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APPENDIX

265

resulting compound will be inactive by internal compensation.
Such a compound cannot be resolved by any process into its
active components. The above compounds may be represented
by the following projection formula?, in which the groups must-
be assumed to occupy three-dimensional space (the asymmetric
carbon atoms being denoted by cross-lines),

1

on
	:ooii
 i r
	coon n- ........ -on

U~
	OH
	on  n

roon
 (/. Tartarit: acid.
		coon
 /. Tartaric acid.

coon
HIo

n

OH

COOH
Raccinic :\r'u\.                                       Mcsotartnric acid.
The conversion of active tartaric acid into the inactive forms
is known as raw what ion, and according to Winther is effected
by the interchange of the gnuips round each asymmetric carbon
atom successively so that part of the active acid is first con-
verted into mesotartaric acid, which then passes into the laevo
variety,
PREPARATION 37.
Citraconic and Mesaconio Acid.The theory of Le Bel
and Van't IIoff has been extended to unsaturated compounds
like fumaric and maleic and the above two acids, which form
isomeric pairs. These two pairs of acids bear a close resemblance.
It has already been observed in the course of the preparation
that citraconic is readily converted into mcsaconic acid. More-
over, they both yield pyrotartnricacid, on reduction, but only one,
citraconic acid, forms an anhydride. Maleic acid in the same
way is easily converted into fumaric acid by bromine, both
maleic and fumaric acid yield succinic acid on reduction, but
only maleic acid forms an anhydride. The explanation is as
follows : in each pair of compounds there exists two carbon
atoms linked to one another by a double bond and each attached
to two different groups. Van't HofF refers the isomerism of each
pair to a space arrangement, which may be represented by
supposing two tetrahedra to be joined by a common edge.
As the centre of each tetrahedron is occupied by a carbon atom,